The mother is "dwindling", receding from life, but still stubbornly living on.
This is clear from the first stanza of the poem:
My mother dwindles and dwindles
and lives and lives.
Her strong heart drives her
as heedless as an engine
through one night after another.
Everyone says, This can’t go on,
but it does.
It’s like watching someone drown.
Literally "dwindling" means becoming smaller, but it's used here to mean that the life in her is becoming smaller. Yet still she lives on, even when everyone thinks she must soon pass away.
The second stanza is devoted to the boat metaphor. My interpretation of this is that the boat represents her body, or more likely her consciousness as you said. It's in poor condition (like the moon shining through the ribs of a boat), and nobody is consciously directing it, but it's not empty of life. Like the mother here: she's not acting or doing anything, but "somebody's in there". Even though she's blind, her sheer force of will is keeping her going: even "blind eyes", with her strength, can still "light her way" towards life.
The third stanza, leading into the fourth, is about the plants in her garden. Even though this can be taken at face value to show how her place is going to rack and ruin without her care, it's also a metaphor. Just as she is deteriorating due to her fading life, so her garden is deteriorating due to her fading influence on it. The metaphorical meaning becomes clear with the line "slurring her edges", coming directly before the topic of her words. The plants don't directly affect her fading life, but this ambiguity and juxtaposition show that they are acting as a metaphor.
The context of this poem is important: it was published in 2007, the year after Atwood's own mother died. She is not just channelling a fictional narrator's daughterly grief, but her own. Also in 2007, she published an obituary which sheds some light on the character of her mother:
When she was 84, she casually informed me that it was time for her to stop climbing up on the roof to get the leaves out of the eavestroughs. "You've been doing what?" I said. She gave me the same stubborn but pleased look she had when, forbidden to shovel snow, she'd have the driveway cleared before you could get there. [...]
She was a believer in doing what you thought was right, seeing things through, buttoning your lip when advisable, and enjoying life as much as possible. She was a permissive mother in many ways - mud pies, frogs in jars, and messy paint held no terrors for her - but she discouraged complaining: Even in her last few years, when she was blind and bedridden, she never did it herself.
This description has advised my analysis above. The mother being described is someone who stubbornly refuses to give in to old age, who sees things through without complaining. She really was blind towards the end of her life, but the "blind eyes light[ing] her way" surely refers to her strength of will that keeps her going even through the impossible.